Seasonal Water Balance in Orthoporus Ornatus, A Desert Millipede

Seasonal H 2 O balance was studied in Orthoporus ornatus (Girard) (Spirostreptidae) inhabiting a volcanic escarpment in central New Mexico. Dormant, subterranean millipedes were collected periodically between November and June; feeding, surface—active millipedes mainly between July and October. For each specimen weights and total body H 2 O, cuticle—tissue H 2 O, gut—tissue and content H 2 O, and remaining H 2 O (assumed to be mostly hemolymph) were recorded. Fluctuation of these parameters for each collection was determined by regressing individual values against corresponding midsegment widths. Total body H 2 O was relatively high in spring and summer; lower in fall and winter. Cuticle—tissue H 2 O remained low from June (time of annual subterranean molt) to early winter, then increased to its maximal value during spring. Gut H 2 O was highest during summer feeding, then declined until after molt. Remaining H 2 O was highest in recent postmolt millipedes surfacing early or imbibing from a moist substrate, and lowest in early winter. Seasonal levels of cuticle—tissue H 2 O and gut H 2 O were similar between sexes at any midsegment width. Total body H 2 O loss was increased by molting and appeared relatively independent of O 2 consumption. Less than half of total body H 2 O loss during the feeding season appeared due to excretion; other avenues were cuticular transpiration and secretions from repugnatorial glands. Total body H 2 O gain was little affected by metabolic—H 2 O production, but gain due to H 2 O ingestion during feeding and to inferred transcuticular uptake during dormancy was considerable. Such uptake should have been active because diffusion gradients of H 2 O activity (a w ) between hemolymph and soil were generally unfavorable to dormant animals gaining cuticle—tissue H 2 O.